The present invention relates to a two-wire communication system, and in particular to a communication system suitable for use in a data communication system or the like between field devices such as terminal process variable transmitters and a control computer of a conventional analog control system used in a plant as well as a present and future digital control system.
As a two-wire data transmission or communication system for industrial instruments such as process variable transmitters, a conventional analog data transmission system is known. In the analog data transmission system, the transmitter controls the current in a range of 4 to 20 mA. In addition to such a conventional analog data transmission system, a system as described in Japanese Patent Unexamined Publication No. 85649/83 is known. In that system, digital data and analog data are alternately transmitted. The digital data communication consists of instructions for setting and adjusting transmitter functions sent from a communication unit to transmitters and answers sent from the transmitter to the communication unit. After the analog data transmission state is replaced by the digital data transmission state, a predetermined delay is introduced and then the analog data transmission state is resumed. This system is effective when a user adjusts and sets the transmitter function in a communication system for analog data transmission. However, the base state of this system is the analog data transmission state, and a system for transmitting the process variable signal as digital data from a transmitter to a receiver or a high rank computer is not described. In addition, the communication unit of the above described system is susceptible to external disturbance or a change in voltage of the power supply because the communication unit is connected in parallel to the power supply, a resistor and a transmitter. Further, the receiver is connected in parallel to a resistor and is different from the communication unit in connection position. This results in lack of uniformity in view of interface to a high rank system in the future.